1. Field of the Invention
The invention relates to a process for the separation of gas mixtures by passing them over an adsorbent bed which absorbs a first component of the gas mixture in preference to a second component of the gas mixture. More particularly, the invention relates to separation of gas mixtures by pressure swing adsorption (PSA) wherein a gas richer than the feed gas in the stronger adsorbed components is utilized to displace the less adsorbable components from the feed end of the bed, whereupon the bed is depressurized by simultaneously taking out gas on at least two different points of the bed.
2. Description of the Prior Art.
First applications of PSA processes were performed to achieve the objective of removing smaller quantities of adsorbable components from essentially non-adsorbable gases. Examples of such processes are the removal of water from air, also called heatless drying, or the removal of smaller quantities of impurities from hydrogen. Later this technology was extended to bulk separations such as the recovery of pure hydrogen from a stream containing 30 to 90 mol percent of hydrogen and other readily adsorbable components like carbon monoxide or dioxide, or, for example, the recovery of oxygen from air by selectively adsorbing nitrogen onto molecular sieves.
The carrying out of the PSA process in multi-bed systems is illustrated by the Wagner patent, U.S. Pat. No. 3,430,418, relating to a system having at least four beds. As is generally known and described in this patent, the PSA process is commonly performed in a cycle of a processing sequence that includes in each bed: (1) higher pressure adsorption with release of product effluent from the product end of the bed; (2) cocurrent depressurization to intermediate pressure with release of void space gas from the product end thereof; (3) countercurrent depressurization to a lower desorption pressure; (4) purge; and (5) represurization. The void space gas released during the cocurrent depressurization step is commonly employed for pressure equalization purposes and to provide purge gas to a bed at its lower desorption pressure.
Similar systems are known which utilize 3 beds for separations. See, for example, U.S. Pat. No. 3,738,087 to McCombs. The faster the beds perform steps 1 to 5 to complete a cycle, the smaller the beds can be when used to handle a given hourly feed gas flow. If two steps are performed simultaneously, the number of beds can be reduced or the speed of cycling increased; thus, reduced costs are obtainable.
U.S. Pat. No. 4,589,888 to Hiscock et al. discloses that reduced cycle times are achieved by an advantageous combination of specific simultaneous processing steps. The gas released upon cocurrent depressurization from higher adsorption pressure is employed simultaneously for pressure equalization and purge purposes. Cocurrent depressurization is also performed at an intermediate pressure level, while countercurrent depressurization is simultaneously performed at the opposite end of the bed being depressurized.
U. S. Pat. No. 4,512,780 to Fuderer discloses a pressure swing adsorption process with intermediate product recovery. Three products are recovered from a pressure swing adsorption process utilizing a displacement step in conjunction with pressure equalization between beds of a multi-bed adsorption system. This process is not cost efficient for the recovery of two products.
PSA processes were first used for gas separations in which only one of the key components was recovered at high purity. For example, from 100 moles feed gas containing 80 moles hydrogen and 20 moles carbon monoxide, the process of the Wagner, U.S. Pat. No. 3,430,418, or of the Hiscock et al., U.S. Pat. No. 4,589,888, could separate 60 moles of hydrogen at 99.999% purity, but no pure carbon monoxide could be recovered; 20 moles of carbon monoxide and 20 moles of hydrogen remained mixed at 50% purity each. Neither of these processes can make a complete separation. Only the less adsorbable, light component is recovered at high purity.
For the recovery of a pure, stronger adsorbed, "heavy" component, an additional step is necessary, namely, rinsing of the bed with a heavy component to displace the light component from the bed prior to depressurization. The rinsing step is described in several earlier patents. The problems with these processes are the following: (a) if the rinsing is complete and the light component is completely displaced from the bed, pure heavy component can be obtained, but the adsorption front of the heavy component breaks through to the light component and the latter cannot be recovered at high purity; (b) if the displacement of the light component is incomplete, the typical concentration profile of the heavy component in the bed as indicated on FIG. 2 is obtained, and if such bed is depressurized countercurrently to recover the heavy key component at the feed end, the light component still present in the bed reaches the feed end very rapidly and the purity of the heavy component drops. Therefore it is not practical with the prior art processes to obtain both key components at high purity in a single PSA unit.
Such complete separations can be obtained, for example, by two separate pressure swing adsorption processing units wherein each unit includes several fixed beds. From a feed gas containing, for example, hydrogen and carbon monoxide (CO), the first unit recovers pure hydrogen and a carbon monoxide rich gas containing 70 percent carbon monoxide. This gas mixture is compressed and passed through a second PSA unit which recovers pure carbon monoxide and a hydrogen rich gas. The hydrogen rich gas can be added as feed gas to the first PSA unit and then the cycle is repeated. The combination of two independent PSA units can make an excellent separation at very high flexibility. For example, from a gas mixture with two components this system can recover more than 99.8 percent of the adsorbable "light" component such as hydrogen at a purity of 99.999 percent and also recover essentially 100 percent of the more readily adsorbed component such as carbon monoxide at a purity higher than 99.5 percent.
A PSA process suitable for the recovery of both the less and more readily adsorbable components is described in British Pat. No. 1,536,995 to Benkmann. The process is based on two beds in series cycle as shown in FIG. 2 of Benkmann. The feed is introduced to the lower bed which retains the more readily adsorbable component. The feed step is followed by a copurge step in which the less readily adsorbable or "light" component is displaced in the lower bed by a recycled stream of "heavy" components, so that the lower bed at the end of the step contains only the "heavy" component. At this moment, the connection between the upper and lower beds is interrupted by an automatic valve and the heavy product is recovered from the lower bed by (countercurrent) depressurization. The upper bed is, in the meantime, also depressurized and purged to remove all of the heavy component. The step sequence of the upper and lower bed are interlocked and cannot be run with independent cycles. The flexibility of this system is therefore reduced while the complexity is increased. Problems with this system are: a set of two beds in series is needed; if process conditions such as feed gas composition change, it is not possible to change the volume ratio of the two beds which means lower flexibility; the vessel heads of the two beds contain more void space gas which increases depressurization loss and compressor power; and the pressure drop is also increased.
There remains in the industry a need to further reduce the amount of capital equipment required for PSA and to boost the efficiency of this equipment. This invention satisfies this need by achieving a high quality gas separation with a simple, more economical and more flexible system. This simplicity and reduced expense is obtained because the invention achieves gas separation at a low compression power requirement. The low power requirement is achieved by the need for less displacement gas. There are known displacement cycles which provide products of high purity; however, the present invention provides such products at lower energy expense.